Nano-silver, as a new functional material, has extremely stable physical and chemical properties. Nano-silver performs excellently in the fields of electricity, optics, antibacterial, antiviral and catalysis, etc., which makes it widely used as catalyst, electrode material, optical material, antibacterial material, antiviral material, and coating, etc. Silver ion is one of the metal ions that have the strongest antibacterial activity, which enables silver ion to destroy the respiratory function of bacteria and result in cell rupture. Such property has made nano-silver one of the most potential antibacterial materials since the silver ions are sustainably released from its surface atoms. In addition, nano-silver also has excellent antiviral activity. At present, it is still unclear how does nano-silver kill viruses, however it may involve the following aspects. Firstly, nano-silver may physically adsorb and immobilize viruses. The surface of ultra-fine nano particles is covered with a layer of polymer with a thickness of 5-10 nm, which can immobilize a large number of proteins and enzymes, especially polysaccharides. Polysaccharides may prevent viruses from adsorbing host cells therefore give nano-silver excellent antiviral activity. Meanwhile, the colloidal stability and strong adsorption of nano particles may deprive viruses of their living environment thus lead to their death. Secondly, nano-silver can prevent viruses from entering the host cells and binding to cell receptors so as to prevent infection to host cells. Thirdly, nano-silver can bind with nucleic acids of viruses to cause the structural change to their DNA/RNA therefore may affect the replication of the DNA/RNA so as to reduce the activity of viruses. Fourthly, the silver ions released from nano-silver may directly kill viruses.
Waterborne polyurethane (WPU) is a new dispersion formed by using water instead of organic solvent as dispersion medium, which is also referred to as aqueous polyurethane or water-based polyurethane. The study on waterborne polyurethane began in 1942, the former West German Schlack added diisocyanate into water in the presence of emulsifier and proceeded to stir intensively to emulsify the mixture. Diamine compounds were then added to perform chain extension, and the waterborne polyurethane was finally obtained. The waterborne polyurethane is environmentally friendly and non-toxic, and has various excellent performances, including abrasion resistance, flexibility and elasticity, etc. The waterborne polyurethane covers all application range of traditional solvent-based polyurethane, meanwhile, it also has excellent physical properties and biological compatibility.
Nano-silver-waterborne polyurethane composite is formed by combining nano-silver with waterborne polyurethane, the properties of which could therefore be studied by controlling the particle size, particle-size distribution and morphology of nano-silver. Unlike those conventional polymer/inorganic composites which are no more than a mix of organic phase and inorganic phase, dispersing nano-silver in organic polymer matrix could give rise to a stronger or a weaker chemical bond (for example, hydrogen bond or Van der Waals force) which is created on the two-phase interface formed by the combination of nano particles and polymers within a range of submicron to nanometer. Because the interfacial area between nano silver and polymers is very large and the desirable bonding behavior of the interface may eliminate the mismatch between the coefficients of thermal expansion of nano silver and the polymer, which therefore endows nano-silver with excellent physical properties and high heat resistance. Meanwhile, the rheological properties of such nano composites in the form of melt or solution are similar to that of polymer, so the nano composites are suitable for various molding processing. Such composites have been widely used in the fields of biomedicine and environmental protection, but have not been explored in the preparation of a latex product.